In manufacturing electronic components such as electronic circuits, circuit boards, resistors, capacitors and IC packages, conductive metal powder is used to form conductor films and electrodes. The characteristics and properties/conditions required for this kind of metal powder include low impurity, fine powder having an average particle diameter of about 0.01 to 10 μm, uniformity in particle shape and particle diameter, little cohesion, excellent dispersibility in paste and excellent crystallinity.
Recently, conductor films and electrodes have been thinner and finer-pitch as electronic components and circuit boards have reduced in size, so that finer spherical metal powder having high crystallinity has been demanded.
As one of methods for manufacturing such fine metal powder, there are known plasma techniques of, after melting and evaporating a metal starting material in a reaction vessel by utilizing plasma, transferring the metal vapor from the reaction vessel to a cooling tube together with a carrier gas so as to cool the metal vapor, and condensing the metal vapor in the cooling tube, thereby obtaining metal powder. (Refer to Patent Literatures 1 to 3.)
These plasma techniques condense the metal vapor in a gas phase, thereby being capable of manufacturing fine spherical metal powder having low impurity and high crystallinity.
FIG. 2 shows an example of a device used in a plasma technique. This is a transferred DC arc plasma device 101 using DC arc, as with Patent Literature 1. The device 101 melts a metal starting material at a crucible part 109 of a reaction vessel 102 so as to form molten metal 108; evaporates the molten metal 108; and transfers the produced metal vapor to a cooling tube 103 by a carrier gas, and cools and condenses the metal vapor in the cooling tube 103, thereby producing metal particles.
The carrier gas is a mixture of a plasma gas and a dilute gas, which is supplied as needed, and usually an inert gas or a reducing gas is used therefor. Examples thereof include argon, helium, nitrogen, ammonia, methane, and a mixture of any of these. A plasma torch 104, an anode 105, a cathode 106, plasma 107 and a dilute gas supply unit 110 shown in FIG. 2 are respectively the same as a plasma torch 4, an anode 5, a cathode 6, plasma 7 and a dilute gas supply unit 10 shown in FIG. 1 described below.
In the case where metal powder is manufactured by a plasma technique, not only for a base metal which is easily oxidized but also for a precious metal which is hardly oxidized, an oxygen gas is not usually used as a carrier gas. This is because problems arise thereby. For example, by introduction of oxygen into a reaction vessel, an oxide film is produced on the surface of molten metal and consequently manufacturing efficiency decreases, or a heat insulating material of the reaction vessel, such as graphite, is burned; and by presence of a large amount of oxygen in the reaction vessel, plasma properties change and become unstable, and consequently the manufacturing efficiency worsens and also plasma cannot ignite in the end. There is also a problem that, in DC plasma, an electrode metal is oxidized and deteriorates.
For these reasons, for example, even in the case where an oxide coating is to be formed on the surface of metal powder in order to improve oxidation resistance and prevent sintering, oxidation has needed to be carried out not by introducing an oxidized gas into a reaction vessel but, as described in Patent Literature 2 and so forth, by blowing an oxidized gas after producing metal powder by transferring a metal vapor to a cooling tube and condensing the metal vapor, for example.